Hackers claim to have cracked a web-based email account used by Republican vice presidential nominee and Alaskan governor Sarah Palin.

The US Secret Service is investigating after a group of hackers calling themselves 'anonymous' managed to break into a Yahoo web mail account Palin was using.

Palin has been criticised in the past for using web mail rather than official government servers to avoid oversight.

"This is a shocking invasion of the governor's privacy and a violation of law," the McCain campaign said in a statement.

"The matter has been turned over to the appropriate authorities and we hope that anyone in possession of these emails will destroy them."

The contents of the email have been published on the Wikileaks web site, although strong demand from web users seems to have crashed the site.

One email seems to come from a member of the hacking group informing Palin that the account had been hacked and telling her to change her password. The email account has since been deleted.

At least one of the emails has reportedly been confirmed as genuine. An exchange between Palin and Amy McCorkell, who Palin appointed to a state drug and alcohol advisory board last year, has reportedly been acknowledged by McCorkell as accurate, although she refused to discuss it further.

"I am reading the paper and have thoughts and prayers going your way ... don't let the negative press wear you down! Pray for me as well. I need strength to 1. keep employment, 2. not have to choose. Lately I just pray may God's will be done," the email reads.